Add lxc-shutdown script

Add lxc-shutdown script

It optionally waits (an optional timeout # of seconds) for the container to
be STOPPED.  If given -r, it reboots the container (and exits immediately).
I decided to add the timeout after all because it's harder to finagle into
an upstart post-stop script than a full bash script.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>